Such an integral structure type milling cutter being integrally composed of a cutting edge portion and a shank is excess of shell-type milling cutter in all the points such as rigidity, machinability, strength, accuracy of finishing, wear proof and the like.
However, when size of a milling cutter itself is large, manufacture of integral structure type milling cutter is restricted, and at the same time the cost therefor is expensive and uneconomical, so that shell-type milling cutter the edge portion of which is exchangeable and which is inexpensive and easily manufactured has mostly been used at the present time.
As a method for attaching such shell-type milling cutter to its cutter arbour, there has heretofore been, as shown in FIG. 7, such a method that a fitting shaft 31a of a cutter arbour 31 is fitted with the inner diameter portion of a milling cutter 11, and further the milling cutter 11 is fixed to the cutter arbour 31 by means of a bolt 5.
In this method, however, when cutting work is conducted, the bolt 5 becomes loose due to vibration derived from cutting, so that degree of fastening of the milling cutter 11 to the cutting arbour 31 becomes poor. In this respect, when rotational frequency or amount of cutting is increased in order to cover such defect as described above, vibration and the like produces this time, and as a result the rotational frequency and the amount of cutting are restricted, so that there is a disadvantage of impeding remarkably improvement of the cutting efficiency.
Furthermore there is also another method for attaching a conventional shell-type milling cutter to a cutter arbour wherein the outer circumference of fitting shaft of a cutter arbour 32 is defined with a left-handed screw thread 32a, whilst the inner diameter portion of a milling cutter 12 is also defined with a left-handed thread 12a as shown in FIG. 8, and the milling cutter 12 is threadedly mounted on the cutter arbour 32. According to this method, when conducted cutting work, the screw is automatically fastened to increase degree of fastening of the milling cutter 12 to the cutter arbour 32, so that high-speed rotation and heavy-duty cutting becomes possible.
In this case, however, the screw is automatically fastened with continuation of cutting work and such cutting resistance resulted therefrom is added as load of threaded portion, so that there are such disadvantages that the threaded portion is completely fastened, in its turn, seizure phenomenon occurs, it makes impossible to separate the milling cutter 12 from the cutter arbour 32, and exchange of the milling cutter becomes impossible.
Accordingly, the present invention has been made to eliminate disadvantages involved in the aforesaid conventional shell-type milling cutters. It is an object of the present invention to provide a method for attaching a shell-type milling cutter wherein an area for fastening the shell-type milling cutter to its cutter arbour is increased to raise rigidity and attachment strength thereby making high-speed and heavy-duty cutting possible, so that the cutting efficiency thereof becomes substantially identical to that of an integral structure type milling cutter, besides exchange of the milling cutter can very simply be attained. The method for attaching a shell-type milling cutter according to the present invention is characterized by fitting loosely the milling cutter through which a tapered hole having somewhat larger diameter than that of a fitting shaft is bored to said fitting shaft of a cutter arbour the extreme end of which is formed into a straight stepped portion, inserting a tapered bush the inside diameter portion of which is extendible in a gap defined between said fitting shaft and said tapered hole, in this situation, inserting a cotter in a cotter hole communicating with the side portions of said fitting shaft, said milling cutter and said bush passing therethrouth, and fixing said milling cutter to said fitting shaft in such condition that said bush is forcibly inserted in said gap between the fitting shaft and the tapered hole.